1. Field of the Invention
The present invention relates to a photosensor array for use with an image processing apparatus such as a facsimile, or a digital copying machine.
2. Related Background Art
It is well known to use a photosensor as a photoelectric conversion element in an image processing apparatus such as a facsimile, a digital copying machine, or a character reader. It is also known nowadays to use an elongated image sensor unit to read an image with high sensitivity. Such an elongated image sensor unit comprises an elongated photosensor array, a light source array for illuminating an original to be read, and an image focusing array for focusing the image of an original onto the elongated photosensor array which is constructed of photosensors disposed one-dimensionally. An example of such a photosensor array used in reading an image is known as a so-called sandwich-type photosensor which is constructed of a photoconductive layer containing such as amorphous silicon (hereinafter referred to as "a-Si"), with a pair of electrode layers being formed on opposite faces of the photoconductive layer. This type of photosensor picks up, as its signal output, primary photocurrent generated within the photoconductive layer upon reception of incident light. Thus, the output signal is relatively small. Further, since the electrode layers are disposed on opposite faces of the photoconductive layer of a photosensor, an electrical shortage may occur if pin holes are formed in the photoconductive layer during manufacturing.
To obviate the above problems, recently a so-called planar type photosensor has been used which has a pair of electrodes formed on a single face of the photoconductive layer containing such as an a-Si, the electrodes being spaced apart from each other by a distance partially defining the light receiving region. This type of photosensor picks up, as its signal output, secondary photocurrent generated in the photoconductive layer. Thus, the output signal is large relative to that of a sandwich-type photosensor.
Known methods of manufacturing an a-Si constituting such planar-type photosensors are the plasma CVD method, the reactive sputtering method, the ion plating method and the like. All these methods utilize glow discharge to accelerate reaction. It is necessary for all of these methods to use a relatively low discharge power in forming an a-Si film having a high photoconductivity and quality. Photoconductive layers obtained at a low discharge power, however, have poor adhesion to a substrate made of, for example, glass or ceramics, which results in a problem that the film is likely to be stripped off during a photolithography process for forming electrodes.
To prevent the film from being stripped off, a method has been heretofore adopted to deposit an a-Si film after making the surface of a substrate rough. Specifically, the surface of a substrate is made rough either chemically or mechanically, such as by using hydrofluoric acid or a brush.
With such a method, the roughness of the substrate is not uniform over the entire surface which has undergone a roughing treatment. Further, microscopic defects often exist locally on the substrate surface. Consequently, the characteristics of photosensor arrays often are quite diverse if they are manufactured in a manner similar to the conventional by depositing an a-Si film on the substrate and forming electrodes.
An elongated image sensor unit constructed of conventional photosensor arrays may produce bit signals of various amplitudes so that correction circuits for correcting such scattering become necessary, which results in a high cost of the sensor unit.